This blog discusses current weather, weather prediction, climate issues, and other topics

Saturday, December 20, 2008

The Storm tonight and Sunday

This is going to be a complex event and I am going to take some time to describe it...and Sunday may end up different than some of the media is describing. I think we can have substantial confidence what will happen through 4 AM tomorrow...after that there is more uncertainty.

Before first, I wanted to comment on the remarkable temperatures we are seeing in western Washington. This morning much of the area has had temperatures in the teens (16F at my house in north Seattle), with many getting into the single digits. One location Arlington got below zero (I can hardly believe it). Here are some examples:

Univ of Washington 19

Mountlake Terrace 13

Lake Forest Park 9

Woodinville 8

S Everett 8

S Whidbey Island 8

Marysville 3

Arlington AP -3

Look at the plot of Seattle temps against normal. Our high temps are less than the normal lows. This is certainly one of the most sustained cold periods in recent memory (please no comments on global warming!--not an issue with a short local situation like this).Another remarkable feature has been the extreme snow in eastern Washington...with records at Spokane and nearby sites.

Now to the forecast. As you can see in the infrared satellite picture at 9:30 AM, the cloud band of with approaching system has reached our coast. The computer models are all in agreement that snow showers will begin in the early afternoon on the coast and move inland, with snow reaching central Puget Sound between 4 and 7 PM. It will snow this evening around here...this is very certain. But before we talk about snow, how about wind?The high resolution computer models are going for strong winds developing this afternoon on the western slopes of the Cascades, the Columbia Gorge, and the Strait of Juan de Fuca. I have attached some of this output for you to see (for 4 PM, 10 PM today, 4 AM tomorrow). The predicted easterly winds along the western Cascade foothills are some of the strongest I have seen, with sustained winds of 50-60 mph with gusts that could reach 70-80 mph. I suspect the model is overdoing this....excessive downslope winds is a frequent model error and looking at its predictions for right now...the observations are less. This will be a very major event...but I suspect sustained 35-45 mph and gusts to 60-70 mph will end up closer to the truth. Very strong easterly winds (sustained 40- 50 mph) will also occur in the Strait of Juan de Fuca. There will be power outages.

Next the snow. As noted above, there is little doubt this Pacific system will make landfall and snow will start in the Puget Sound lowlands sometime later in the afternoon or early evening. There is also little question there will be a large east-west snow gradient over lowlands with heavy snow in the Kitsap and eastern slopes of the Olympics (1-1.5 ft), moderate snow near the Sound (3-8 inches), and lighter amounts towards the western foothills (1-3 inches). As noted in yesterday's blog this difference is due to the easterly flow...the resulting downslope flow over the western Cascade foothills produces drying and upslope on the Olympics causing increased precip on its upstream side. Attached is the 24h snowfall ending 4 AM. You see this features and others...such as the heavy snow in the mountains...and substantial snow over the eastern Strait. There is also a rainshadow (or snowshadow) north of the Olympics. Since I think the easterly downslope flow is being overdone a bit...as is the drying effects, I suspect that the snow amounts over Seattle and the east side are being underdone in the model, and this could increase the snowfall by a few inches. So here we are around 4 AM with lots of snow. The cold front associated with this system will move through in the morning and there will be increased flow from off the ocean...and as a result there will be some warming aloft. A secondary trough of low pressure moves through during the afternoon on Sunday with more showers. These will bring snow in the mountains, but what about the lowlands? That is where this gets complicated and more uncertain. Looking at the temperature predictions it appears the central Puget Sound and NW Washington could stay as snow...albeit wet snow and we could end up with several more inches. The coast will switch to rain as will parts of SW Washington. I don't see a major freezing rain threat in the PS lowlands.But let me be clear...I am less sure about the second half of the forecast than the first part. Anyway, be prepared to see a notable weather show.

Any thoughts regarding Sunday? You hinted that it might be tricky to call. This latest entry ended after the wind charts so maybe something got cut off? At any rate, I am very thankful for the content you've provided.

Are you guessing that the wind event will be as severe as the one in 2006? It sounds ominious given your projected wind speeds. If I recall, it's from a somewhat different direction than the one in 2006, so trees that survived that one may not be so lucky this time (if it is as severe). Any thoughts?

I'm more concerned about the wind overnight and tomorrow morning. I live in downtown Bellevue (along the I405 corridor)and was without power for 5 days during the wind storm a couple of years ago. How likely will Bellevue be hit with those 60-70 mph wind gusts that are predicted for the foothills?

I live out in Duvall and it seems that we are in a bit of a gray area for the winds and snow. We are in between the "gaps" but farther east than Woodinville. Could you possibly give a brief description on what we could expect out here? Much appreciated!

Hi Cliff - Being here in Duvall too, I ditto Tim's post. I assume the "Cascade foothills" pertains to the 'shoots' down the passes affecting I-90 and Hwy 2 corridors moreso than here in the Snoqualmie Valley.

Cliff, thanks for your thoughts on this. Based on your models and the NOAA forecast, we have pre-emptively cancelled our church service in Freeland tomorrow morning. Whidbey Island is hard to predict but we'd rather be safe than sorry.

Wind is starting here in Maple Valley. Hope it is not as strong as the Dec '03 wind storm was. It looked like a tornado hit here. I watched 150' firs splashing down into lake wilderness, and many other trees were tossed like matchsticks. Power was out for me for seven days, as opposed to only five in the storm of '06.

Great blog Cliff!I live on a high bluff on Mutiney Bay (Whidbey Island) and have a 180 degree view from the entrance to Hood Canal to Bush Point so I not only have a great view of the water and marine traffic but also the Olympic Mtns (which are fading as I type). I'm not at all an IT person but I would like to know what it takes to mount a camera & basic weather station for wind speed/direction and temerature. It is often off from what the various weather news is reporting. It would also be cool to look at in real time (plus great sunsets).Thanks!

First of all, you were my first professor at UW in 1997, Atmos 101. It's so great to still be able to benefit from your sage wisdom long after I graduated! Thank you for writing this blog.

The thing I can't stop wondering about in all of this is the cold air issue...apart from the wind event tonight, it seems like the single most disruptive element to all of this is the utterly persistent low level cold air. I grew up here, and I know we get cold snaps, but it just doesn't seem normal for these extremely cold temperatures to continue day after day after day like this. It seems like the city's geographic location generally shields us from prolonged cold events. And yet the forecasts through at least mid next week presently indicate that we will continue to hover just around freezing. In Seattle of course, freezing means continued impassable roads, threats to power lines, etc.

Isn't there any kind of element out there that could help scour out the cold air at the lower levels of the atmosphere? Southwest winds, warm front, anything?

No one seems to be commenting on the issue of all the snow still in the trees. It would seem to me that with all that weight in the trees, they will come down a lot easier. Does frozen ground help out in any way ?